1. Field of the Invention
The present invention relates to a filter arrangement for suppressing an AC voltage superposed on a DC voltage provided across first and second poles of a voltage source.
2. Description of the Prior Art
Such a filter arrangement is already known in the art, e.g. from the published European patent application EP 0363602A1. Therein, the voltage source includes a primary DC voltage source connected to the first and second poles via a series regulator regulating the DC voltage across these poles. This series regulator includes the series connection of a regulating transistor and a resistance between the primary DC voltage source and the first pole, and the regulating transistor is controlled so as to provide a predetermined voltage across the first and second poles. The filter arrangement includes a first capacitance which acts as a capacitive sensor for picking up from the second pole the AC voltage or noise voltage superposed on the DC voltage. This picked up AC voltage is then amplified by an operational amplifier providing a compensating voltage which is applied to the junction point between the regulating transistor and the resistance via a second capacitance preventing a DC voltage at the latter junction point from reaching the output of the operational amplifier. Thus, an active filter arrangement is obtained whose efficiency is determined by the values of the components used, i.e. mainly by the gain of the operational amplifier and the value of the resistance. Indeed, these values must be so chosen that the above compensating voltage is exactly opposite in phase and equal in amplitude to the AC voltage initially present at the junction point. This means that the filter arrangement is sensitive to variations of the values of these components.
To be noted that the operation of the above filter arrangement may also be appreciated from the fact that it is based on the insight that the operational amplifier injects at the above junction point an AC current which is equal in amplitude and opposite in phase to the AC current corresponding to the AC voltage superposed on the DC voltage present across the first and second poles. It may then also be appreciated that the amplitude of the AC current to be suppressed should be smaller than the DC current corresponding to the DC voltage in order to be able to effectively suppress it. When the DC voltage is applied to a so-called travelling wave tube (TWT) operating with a current which is typically equal to 0.1 milliampere (mA) to 1 mA whereas the AC current to be suppressed may well be equal to 1 mA to 2 mA, this AC current may thus not be effectively suppressed. Accordingly, in order to be able to be effectively suppressed the amplitude of the AC voltage to be suppressed should be relatively small.
An object of the present invention is to provide a filter arrangement of the above known type but which is less sensitive to variations of the values of its constituent components, and which puts no constraints on the amplitude of the AC voltage to be suppressed.